Slashdot videos: Now with more Slashdot!

View

Discuss

Share

We've improved Slashdot's video section; now you can view our video interviews, product close-ups and site visits with all the usual Slashdot options to comment, share, etc. No more walled garden! It's a work in progress -- we hope you'll check it out (Learn more about the recent updates).

ambermichelle pointed out a story about the search for life on other planets, and the likelihood that it would be much different than what we find on Earth. With the increase of extremophile discovery in recent years perhaps it's time to reassess what the definition of "life" should be. "In November 2011, NASA launched its biggest, most ambitious mission to Mars. The $2.5 billion Mars Science Lab spacecraft will arrive in orbit around the Red Planet this August, releasing a lander that will use rockets to control a slow descent into the atmosphere. Equipped with a 'sky crane,' the lander will gently lower the one-ton Curiosity rover on the surface of Mars. Curiosity, which weighs five times more than any previous Martian rover, will perform an unprecedented battery of tests for three months as it scoops up soil from the floor of the 96-mile-wide Gale Crater. Its mission, NASA says, will be to 'assess whether Mars ever was, or is still today, an environment able to support microbial life.' For all the spectacular engineering that's gone into Curiosity, however, its goal is actually quite modest. When NASA says it wants to find out if Mars was ever suitable for life, they use a very circumscribed version of the word. They are looking for signs of liquid water, which all living things on Earth need. They are looking for organic carbon, which life on Earth produces and, in some cases, can feed on to survive. In other words, they're looking on Mars for the sorts of conditions that support life on Earth. But there's no good reason to assume that all life has to be like the life we're familiar with. In 2007, a board of scientists appointed by the National Academies of Science decided they couldn't rule out the possibility that life might be able to exist without water or carbon. If such weird life on Mars exists, Curiosity will probably miss it."

If you know anything about TV science fiction, then you would know that all sapient life forms look like white people with maybe some ridges on their forehead or something (and they speak English). All flora looks just like what you find in California. And animals look like shambling people in horribly fake costumes.

I'm a biologist, and tomorrow I'm leading a discussion section on "What is life" (first class of freshman biology, starting out easy).

There IS no set definition of life. Viruses, prions, crystals, fire, mules, computers, you can come up with obvious exceptions to any criteria. Reproduction? Fire does that, crystals do that, mules do not. Metabolism? A car battery undergoes some metabolism, bacterial spores and seeds I think don't, though I could be wrong.

The closest thing we have is like Justice Potter Stewart's definition of porn: we know it when we see it.

Without disputing that there is no consensus on the definition of life, I am of the opinion that there is a possible definition which includes everything we take to be alive and nothing we reasonably shouldn't take to be alive. It is in terms of thermodynamics, specifically entropy.

Let us define mechanical work as "productive" upon some system when it changes the said system to a state which is less entropic. We can then say that a machine X is productive upon some other system Y when the product of its work is a decrease in the entropy of Y. There may be limited circumstances under which X is productive upon Y; to do work X will need some sort of energy flow through it, but not any energy flow will do. So for example, an electric machine which sorts and stacks coins coined can be said to be productive upon the coins when it is plugged in to an electrical circuit of the proper frequency and voltage, and turned on, and otherwise in its operating conditions, but not just when it is being heated, say.

With that out of the way, we can now define a system as "alive" when it is productive upon itself, or more simply define "life" as "self-productive machinery". The given conditions under which a given system is self-productive will of course be limited and vary, but those constitute the conditions under which a given system can live, which are also limited and vary.

Crystals, fire, and car batteries all seek lower-energy, higher-entropy states, and so are not life, though they might fuel life (like fire) or be instrumental in the construction of living things (like crystals).

Viruses, by this definition, can only live inside of more complicated cells, the way that a severely deformed baby might only live on life support, or humans in general can only live in an atmosphere of appropriate composition, temperature, and pressure, with sufficient water and appropriate chemical fuels available. A virus floating around by itself is dead, though under the right circumstances it can come to life (unlike humans, but that's because dead humans decay more readily than dead viruses, being big complex things instead of simple molecules). Spores and seeds likewise: not alive just sitting there inert, but alive when put into the conditions in which they grow. Prions I don't know enough about to say.

Mules are definitely alive, whether or not they are a viable species; reproduction is just one way for life to continue living, not a prerequisite for living at all.

The only really controversial part of this definition is that computers may count as alive, when plugged in and turned on etc, the same way that viruses may count as alive when inside another cell of the appropriate type. But that's only controversial because they are artificially constructed machines made from something other than the stuff we are made of. Artificially constructed organic nanomachines modelled after the ones we're built on are indisputably alive: artificial life, but life nonetheless. Computers differ only in being bigger, and made of different materials. Their information-processing functions certainly reduce the information entropy of their storage media.

This factor has the nice benefit to this definition of ensuring that intelligence, sentience, sapience, etc, are a proper subset of life; you can't have something which takes in information about the world around it and does something productive with it, without that thing being alive by this definition in the process.

A mule strikes me as a cheap argument. They may be unable to reproduce, but their ancestors stretching back 4 billion + years were able to. By that argument you could castrate a man and say 'look, your definition of life is invalid'.

Fire does sorta throw water on the definition, though. It definitely "feeds", and reproduces in the sense of expanding. So do crystal formations, which aren't considered alive.

I'm not a biologist.

I think you can distinguish fire vs. life by metabolism. With metabolism, chemical processes inside the organism occur to fuel the organism. With fire, those processes do not happen within the flame, they happen inside the fuel. I can't think of an organism which fuels itself by chemically transforming fuel outside of its "body". This may be shaky, because you could argue that the chemical processes would not happen were it not for the presence of the flame. Fire doesn't sound like it fits the definition of metabolism though, unless you view the flame as simply the result of the metabolism and the organism itself which produces the fire through metabolizing the fuel is unknown.

With crystals growing in a liquid, matter which is dissolved in the surrounding liquid gets added to the crystal. There is no real chemical change that takes place, the dissolved particulates just coalesce on something else. When the liquid and its particulates is removed the crystal no longer grows, but the crystal itself does not take the particulates out of the liquid. The particulates simply adhere to the crystal (or any other structure on which the crystal starts).

I can't think of an organism which fuels itself by chemically transforming fuel outside of its "body".

Cooking, or other food processing, could fall under this broad concept.

This whole debate is silly, just like the definition of planet that people were arguing over a few years ago---people who should be doing science and not trying to define and then redefine words to re-fit reality into their preconceptions. Definitions are human constructs, attempting to create bright lines between what is an x and is not

With crystals growing in a liquid, matter which is dissolved in the surrounding liquid gets added to the crystal. There is no real chemical change that takes place, the dissolved particulates just coalesce on something else. When the liquid and its particulates is removed the crystal no longer grows, but the crystal itself does not take the particulates out of the liquid. The particulates simply adhere to the crystal (or any other structure on which the crystal starts).

Hmm... That needs a bit of rewriting. It reads a whole lot like a description of most of the higher plants growing out in our garden. They require much of their nutrition from the water that pervades their soil (and they die if that water is removed). They even get their carbon from the CO2 that has dissolved in the water in their tissues; they don't take it directly from the atmosphere. They absorb the nutrients from the water, transport them via diffusion and capillary action (two things that are n

Yup, these discussions are dumb, like debating whether Pluto is a "planet" or not. Life is an ascribed property, so of course it has no crisp boundaries in nature. Some things clearly are, others clearly not, and a few things are very debatable. Find new phenomena on earth or other planets first, then we can judge how interesting they are and whether they alter our notion of "life."

1) What sort of life-detection-capabilities should we build into a mars lander?

2) What is the permanent fundamental definition of "life"?

The first is pretty easy to answer - we've got limited resources, so it makes sense to look for the kind of life we know how to look for. True, we might miss other things that (had we not missed them) we'd consider "life" - but what choice do we have? We don't know what "different life" might look like, so we can't b

Meh, not so simple. I won't even talk about things like mules, or other infertile animals; but are for example erythrocytes (red blood cells) alive? They feed, but don't reproduce, you know; how about viruses? They do reproduce, but surely don't feed. Or prions? They also reproduce, but don't even carry nucleic acids. What about Sydney Fox's protobionts [wikipedia.org]? They both (kind of) reproduce and (kind of) feed, but can form spontaneously from inorganic matter.

Sure, life in the universe COULD be different than our carbon-based, water-needing forms. But there are restrictions on how many detectors etc. you can package on one rover. Given that difficult decisions need to be made in regards to equipping our search for life, it makes sense to search for life in a form that we are 100% sure exists at least one place in the universe.

Exactly this--there might well be other forms of life, but we only really know how to look for life like our own.
You may say that it's dumb for NASA to look for carbon-based life, or for SETI to look for life that uses radio wavelengths like us, but if you do so you're misunderstanding their logic. If there is enough life out there, some subset of it will be carbon based, some subset will use radio communication, and some subset will be interested in communication. That subset is the ONLY subset that we have the tools to look for. There may be non-carbon-based life, sure, but since we've never seen it we don't know exactly what its properties are or how to detect it. We may be able to theorize, but those are only theories; whereas we KNOW how life works here.
It's not that researchers have a narrow definition of life, it's that we have limited resources and can only hope to detect the subset of life that is like life here on Earth.

After all, what you call 'life' is just a definition of complex chemical constructs that can propel themselves and add to their structure in a consistent fashion. That is the basic definition of life. They are basically systems.

And these constructs totally depend on the greater system they are part of. All the conditions, present compounds and elements in a given environment, would cause any such self developing and propelling systems to evolve shaped according to that environment. it does not necessaril

Overly inclusive perhaps, but life could generally be defined as the ability to actively resist entropy (maintain low entropy) coupled with a method of passing that ability along. You could say that crystal structure represent a low entropy state, but they have no method to actively propagate it or pass it along other than growing. Throw out counter arguments at will, but I say it's pretty good.

Here it is:Three of them were big enough, as planets go, to be noticeable; the rest were mere pebbles, concealed in the fiery skirts of the primary or lost in the black outer reaches of space. All of them, as is always the case, were infected with that oddity of distorted entropy called life~, in the cases of the third and fourth planets their surface temperatures cycled around the freezing point of hydrogen monoxide-in consequence they had developed life forms similar enough to permit a degree of social co

I am always depressed about the primitivity of human thought, when I hear people discuss "Is this alive, or is it dead?". As if that was some binary either/or question or switch.We have to face, that for every step between completely dead and whatever we define as completely alive, there exists something that fits that. And why wouldn't there?

Then we can rethink our egocentrism, and accept that we are neither special nor unique, and that that is OK.It really is.Life has to follow the constraints that we defined it to have, and therefore logically is between some bounds. E.g. the elements it uses, if it needs water, what temperatures it requires, what processes it uses and consists of....But really it's just a definition thing. And nothing else. Since "life" is just a word. Nature itself does not know the concept of a "concept".:)

So I see this from a relaxed point of view. All this bickering about definitions and "ME, ME, ME, ME, ME, ME!" doesn't matter.What matters, is that we are on the brink of discovering things on other planets... Things that can be so vastly different from ourselves, that our knowledge may leapfrog forward... And that yet may be so very similar to us in so many aspects, that is will tell us things about ourselves we could never have imagined.

IF we're going to find any life on Mars, it's probably going to be the carbon stuff we've been hearing so much about. Silicon life and other sorts of voodoo biology might exist in stranger environments but Mars is basically a big dry dust-ball sitting next to a big wet swamp-ball. Odds are that whatever splashed our planet in the first place also got Mars, and Mars just so happened to be tinier, lighter and colder than us enough that its water cycle kind of evaporated. Or at least that's the theory they'

We don't assume something just because we can't rule it out completely, we assume something because there are signs indicating it's true. We have pretty good proof that shows that carbon-based life can exist, but there is neither physical nor theoretical proof of other exotic lifeforms. Not being able to rule it out is not enough reason to send another expensive probe when that money could finance far more promising research.

Why would we care? We have a hard enough time communicating and getting along with those beings who we share 99.9999% of the same DNA with. Imagine trying to talk to some blob of silicon that is trying to say hello with ionizing radiation.

They are looking for organic carbon, which life on Earth produces and, in some cases, can feed on to survive.

This is likely to trigger red flags in the minds of a lot of people with biological training. Just what is "organic carbon"? That's a media phrase that isn't too well defined in scientific circles. There's a great variety in the "organic" carbon chemistry of our world. But we should expect that any life on other worlds, even if it uses carbon, will produce compounds and radicals that are different and/or more varied than what we see here.

Another problem is that astronomers long ago pointed out a probable path for Earth bacteria colonizing the rest of our solar system, and possibly beyond. Earth has a thin "dust tail" produced by the same solar light pressure that produces comet tails. This is a problem for some kinds of astronomical observations in the plane of the solar system, since our dust tail reflects back back to us. Anyway, back in the 1970s, satellite and upper-atmosphere probes verified the presence of both fine dust particles and bacterial spores at all altitudes. The planet's dust tail thus contains such dust and spores. So the Earth has been contaminating the outer solar system with bacterial spores, presumably for some billions of years. We don't know whether any of those bacteria can survive on the outer planets. But the default assumption should be that some of them have, and have adapted to some degree over those billions of years to their new environments. Maybe they have; maybe they haven't. But if we find Earth-like bacteria out there, they probably came from here.

Some astronomers have also calculated out that part of our dust tail (and comets' tails) escapes the solar system. So we've been contaminating the galaxy with bacterial spores for billions of years. A billion years is around 4 or 5 orbits of the galaxy, up to 20 or so orbits since life arose here. The chaotic nature of galactic dynamics mean that our dust has spread through the entire galaxy, as has the dust from other planets with atmospheres.

This argument is more often used by the "panspermia" supporters, who point out that life from anywhere else in the galaxy could have colonized Earth in its early years, since the galaxy is around 13 billion years old, while our solar system is only about 1/3 that age. But some astronomers use it to explain how earthly life could have colonized the rest of the galaxy before humans evolved here. And, of course, both could be true.

Of course, the main problem with all this is that we have no data on how well bacterial spores can survive the millennia in interstellar space. Probably not well, but it doesn't take a whole ecosystem to establish a colony. For bacteria, it only requires one spore (and hundreds of millions of years;-).

Probably the best prediction is that eventually, some probe will find a few bacteria on Mars and/or other planets, and they'll be somewhat similar to bacteria on our planet. This will raise more questions than it answers, as is common in most scientific fields.

James Lovelock came up with a perfectly good definition that doesn't stipulate any specific chemistry - he merely stated that life is that which will actively sustain a dynamic equilibrium when the non-living parts of the system passively change*. (He also argued that the distinction between living and non-living was stupid anyway, since there are too many inter-dependencies to make such a distinction in a productive way. Since his work forms the backbone of almost all modern life science, it seems pointless NASA resorting to definitions of "life" that have been considered obsolete for a decade or more.)

Indeed, Lovelock's theories on life are exceptionally useful to astronomers, because you CAN monitor the chemistry of the atmosphere of an exoplanet and you CAN monitor things like the solar radiation it gets. You can therefore utilize Lovelock's work to determine if the planet has life on it or not, remotely, without any regard whatsoever to the chemistry of that life or the mechanisms it utilizes.

*The basis of Lovelock's definition is that all life MUST geo-engineer. It has to, with no exceptions. That goes for viruses, bacteria, algae, etc. Not only must it geo-engineer, but in order for a system to be in dynamic equilibrium, the geo-engineering HAS to contain a negative feedback loop. The mere presence of life will alter the planet, but if it were to alter it without creating a dynamic equilibrium it would necessarily create a positive feedback loop that would destroy itself. In his view, you cannot treat the geology, the meteorology and the biochemistry as distinct fields - they interact and compartmentalizing will never let you understand the processes going on.

Analyzing soil samples will help on Mars but really it shouldn't be necessary. Dormant's another matter. If life exists in an active form, there will be variables that are held to a value and do not passively fluctuate with the seasons. If life *ever* existed on the planet, then the chemistry of the rocks will show that variables were held to a specific value and did not fluctuate with the seasons. The geology will record the feedback processes that all life (in this model) must have. The soil samples would let you identify what that life was/is, and to understand HOW it operated, but to merely detect if it was there to begin with you need look no further than the chemistry of the sedimentary rock we already know exists on Mars.

That is, if his theory is correct.

Evidently, despite the views of the life sciences, NASA is not following this path. Ergo, NASA thinks that despite the fact that it doesn't know what to look for, it shouldn't look where Lovelock said. I would hope they have a really good reason -- it's exceptionally bad science to ignore the prevailing theory, particularly if you have none of your own. They have to be rejecting his theory because if they accepted it then they wouldn't need to care about carbon, water, etc. They'd merely need to care about whether the chemistry could or could not be explained by passive processes alone. What the process was would simply not matter.

If we focus on planets in "the habitable zone," then life is much more likely to be based on C, N, O, etc. than something more exotic (to us) because that's likely to be the prevailing chemistry able to generate "self-reproduction with variations." Many of the other alternatives with Si, liquid methane, or other weirdness is unknown to us because such chemistry is extreme within the context of the habitable zone. Next questions: Is Mars in the habitable zone? Is there enough water, atmosphere, background energy, and other conditions needed to generate sufficient quantity and turnover of organic compounds to sustain self-reproduction with variations?

How we define "life" when searching the cosmos is entertaining, but to me the bigger philosophical question to consider is alien sentience (not just intelligence) -- self willed, thinking, rational or irrational beings who think, feel, and act for themselves similar to us, but likely following completely different structures of society and morality.

After posting the article linked in the summary, Zimmer followed up by posting the comments of evolutionary biologist David Hillis [discovermagazine.com] on his own website. For those that don't want to read the entire post, the basic idea is that we ought not try to define life as a collection of characteristics (i.e. reproduction, inheritance of traits, existence of metabolism, &c.). Any such definition is likely to exclude things that we think of as alive, or include things that we think of as not-alive. Instead, it ma

If we wouldn't recognize a different kind of life on Mars, would we also not recognize it on Earth? We should expect that some kinds of life on Mars have also arrived on Earth several times, and that Earth life has also reached Mars several times. It's unlikely, but there have been quite a few rocks thrown into space from both planets. So maybe the same speculated odd life is already here, but we don't know how to recognize it no matter where it is.

Science doesn't have a definition of life to rethink. The best we have come up with is: "Living organisms undergo metabolism, maintain homeostasis, possess a capacity to grow, respond to stimuli, reproduce and, through natural selection, adapt to their environment in successive generations. More complex living organisms can communicate through various means.[1][5] A diverse array of living organisms (life forms) can be found in the biosphere on Earth, and the properties common to these organisms—plants, animals, fungi, protists, archaea, and bacteria—are a carbon- and water-based cellular form with complex organization and heritable genetic information." (wikipedia)

This is not a definition. It doesn't even claim that all conceivable or possible things that have these properties are life, and I would not discount the possibility of finding something that did not have one of these properties that still seems worthy of calling life. Maybe science should change its definition of asjkdhljkfg while we are at it. The 'definition' doesn't even say that life is carbon based or water based as the summary seems to suggest, rather it goes out of its way to stress that this is just true for what we have seen so far.

If you substitute the term "life" for something like "Earth-like life," all of the quibbling goes away. There absolutely, certainly could somewhere be life that is not at all like the life we know of on Earth, but we have no idea what that would look like or how to detect it for the first time, so it makes no sense to look for it right now. We know what life on Earth looks like, and how to recognize it (or the conditions that it requires and in which it is found everywhere on Earth), so it doesn't make sense, at this moment, to look for anything else, or not to look for conditions that favor Earth-like life. Hopefully someday we'll be cognizant of non carbon-based life forms, or life that does not rely on water (or watery conditions that for whatever reason do not support life), but for now they're doing things right and acting prudently rather than wasting precious resources on wild goose chases.

On my planet, 'replicating' involves producing identical offspring (or nearly so.) If your method of reproduction involves a continual reduction in mass, we may need to rethink how the dictionary works.

In all seriousness, though, the definition of 'life' taught to young scientists doesn't proscribe any particular construction materials; hence this article (or at least this summary) is deceptive. The requirements are:

1. Homeostasis. It must make a detectable effort to maintain the conditions of its internals, and to adjust to changes in its environment.
2. Reproduction. It must be capable of creating copies of itself (or approximate copies of itself.)
3. Evolution. Its offspring must be able to adapt to changes in the environment through to natural selection.

That being said, there are circumstances in which some of these are suspended, like ancient trees and soldier ants that can't reproduce but are most definitely alive. The maintenance of an internal environment (homeostasis) is considered the most important, and the primary reason scientists have hesitated to consider transposons and viruses to be alive, even though they can reproduce and evolve.

Outside of these guiding principles, though, biologists really have no problem with the Enterprise running into plasma filament creatures, or Doctor Chaotica's henchmen duking it out with photonic life forms (although physicists might.) We're very good at pointing out flaws with some of these ideas (like "silicon is extremely bad at supporting life when compared with carbon") but that doesn't mean chemical evolution will never find a way to do it anyway.

I have rarely seen a baby that looks just like an adult. And after the birth, both parent and child are smaller than the combined entity prior to the birth. The real difference vs the broken rock shows up in what happens later.

1. Homeostasis. It must make a detectable effort to maintain the conditions of its internals, and to adjust to changes in its environment.
2. Reproduction. It must be capable of creating copies of itself (or approximate copies of itself.)
3. Evolution. Its offspring must be able to adapt to changes in the environment through to natural selection.

I'm not a biologist but I enjoy learning so I have a some questions about these definitions.

What's the current thought on virus? Are they 'living'?

Concerning #2. Shouldn't life have to create approximate copies? If they create [exact] copies, wouldn't that negate #3?

Concerning #3, If life doesn't make exact copies, doesn't evolution have to happen by way of natural selection. In other words are #2 and #3 redundant?

We classify viruses in the same way we classify living organisms, but there's still a lot of debate about whether or not they're alive. I could come up with a half-baked underslept computer analogy, but just going to Wikipedia [wikipedia.org] would probably be more useful.

Regarding #2: a truly reliable and perfect form of biological reproduction is asymptotically impossible due to thermodynamics (this is mentioned in the article.) Assuming 'nearly perfect' = 'perfect', I meant 'approximate' to refer to complex mechanics like sexual reproduction, where the traits of multiple parents are mixed, and random evolution is enhanced.

Regarding #3: It means the organisms produced by mutation must be sufficiently different for natural selection to act upon them. A photocopy machine operating repeatedly in the absence of humans will produce imperfect copies, but no one cares.

a truly reliable and perfect form of biological reproduction is asymptotically impossible due to thermodynamics (this is mentioned in the article.),

I am not a physicist either:) but while it is in the article, the author merely mentions that the laws of thermodynamics make it impossible. But presuming he is referring to the second law and entropy, it only applies to isolated systems so I wouldn't think it's impossible.

I'm not trying to be argumentative I just find that discussions on/. with informed people can lead me to a better understanding of things.

I think this is actually a physical chemistry talking point, so don't feel too bad.:) When a chemical reaction occurs, very rarely is it an instant on/off thing caused during a single, instantaneous collision. Most reactions take a number of steps, each of which has a certain probability of occurring. Misreactions also have a small probability of occurring, albeit generally lower; in organic chemistry every reaction has a percent yield and needs purification afterward (an imperfect process.) Because of all this, reaction as complex as a biological enzyme binding, modifying, and then releasing its substrate can take many, many attempts (I don't have the magnitude on hand, but you can bet it's many times larger than a billion molecular vibrations) before it occurs, and is never perfect. To make things worse, the reproduction of DNA requires multiple enzyme reaction steps per nucleotide, and one of the steps is responsible for verifying that the nucleotide being inserted is correct. There are additional steps on top of things that try to do proofreading, but since everything is error-prone, the whole process can, ultimately, fail.

I prefer a simpler definition: universal machines, in the sense of the Church-Turing thesis. Of course, we say that computers are not alive. But we define life in a way that excludes computers, at least, current computers.

It is surprisingly easy to support universal computation. One might think it takes all kinds of complicated logic and machinery, but this is not so. Some two input logic gates, such as NAND, are enough. Conway's Game of Life is a simple cellular automaton that can do universal comp

While that definition certainly encompasses all of what biologists consider to be life (and I've pondered it, too,) it misses several properties of living organisms that we find interesting to study. Life is different from a computer in that it is also partially defined by what functions it computes, not merely that it can compute anything; these functions are those that lead to an increase in its tendency to proliferate and persist in its environment for a longer period of time. For a biologist to consider

There are always exceptions to everything. Science isn't about what happens all the time, it's about what happens 95% of the time and hoping that we're right. For all we know, our definition of "gravity" is wrong and it has nothing to do with actual mass but something else that we can't detect that generally corresponds with mass.

Essentially, what we're arguing is semantics. When we say "life" we can't even be sure what we're talking about without using a sentence to specify. We could be talking about any n

Well, that's what I get for oversimplifying things for the Slashdot audience and not remembering lectures verbatim from four years ago. But you may want to take your ad hominems out back and shoot them: the Wikipedia page is somewhat more thorough [wikipedia.org], and includes organization, which is the critical quality that rules out fire. To be living, an organism must do all of these things (evolve, adapt, reproduce, respond to stimuli, and maintain its internal environment) through orderly, controlled means. In standard organic Terran terms, that means metabolic chemical pathways.

And for your information, the exceptions I listed aren't exactly classic exceptions. The question of whether viruses constitute life is under debate [wikipedia.org], and sterile organisms are essentially modifications of other members of their species, which are very much capable of reproduction.

Finally, the definition is supposed to be used to differentiate large groups of phenomena from life, and has widely been recognized as inexhaustive and incomplete for a long period of time. You expect too much of experimental science if you believe that a scientific definition must be so rigourous.

If their heritable mutation affects their probability of reproduction, so that it is capable of evolving in a Darwinian fashion. There is a theory that the earliest form of life might have been crystalline.

If your computer reproduces, and has heritable mutations that affect reproductive success, so that it is subject to Darwinian evolution, then I would consider it to be life, whatever the its form or behavior.

The answer to the question, "what makes us humans different from animals?" is of course "nothing," because we are animals. If you mean, "what makes us different from other species of animals?," the answer is different genes.

If you ask, "what about our behavior is different from other animals?" there is

People like you make me fear for my unborn grandchildren. I see folks with no understanding of how computers work thinking that they're "thinking machines", like idiots in the media have been calling them since ENIAC, which was less powerful than a Hallmark greeting card.

A computer is nothing more than an electric abacus. It works on exactly the same principle as an abacus. IT DOES NOT THINK, IT IS NOT ALIVE! I made a pseudoautonomous program thirty years ago on a machine with only 16k of memory and no disk

They are highly evolved to a specific environment, but are simplified often to an absurd degree as they do away with unnecessary organs and structures...

They do not retain information they deliberately lose it, they are not "higher" except in the sense that they are as efficient as possible (which is all evolution produces, a best fit for an environment)

Perfect replication + deliberate error mechanisms = evolution. Evolution has a tolerance rate; too many mutations too quickly and your evolutionary magic turns into lethal dysfunction. The rate of evolution for E. coli, for example, is a few orders of magnitude smaller than 1/(the number of nucleotides), which means that most of the time the offspring are a perfect match. Relatedly, humans manifest a substantial number of new point mutations when the gametes are formed, but have a much lower rate when producing somatic cells through mitosis. It's replication with a very small p-value. The article discusses the thermodynamic inevitability of mutation, if you're genuinely interested.

There's nothing deliberate about the error mechanisms that cause mutation and therefore make evolution possible. "Deliberate" is the kind of word that's more appropriate to discussions of "intelligent design" than evolution.

The error mechanisms exist and are arguably necessary, but in the world of chemistry, randomness is everywhere so you don't need to go looking for deliberate errors.

Actually, theoretically, the definition of 'alive' is disputed, because some scientists want viruses to be 'alive' and some don't. (Among many other disputes as to what the formal definition should be).

But parasites, even internal ones that rely on the environment of their host bodies to reproduce, take in food and process it into new parasites internally. For a virus, everything that could be considered a life process is outsourced to the host. A virus, at least by a classical textbook understanding (although I distrust those because they're always oversimplified), doesn't have any life processes of its own. It doesn't aquire food or process it to grow/heal/regenerate, or to make more viruses. It doesn't